1. Field of the Invention
The present invention relates to the automatic measurement of seed melt back of a produced crystalline material.
2. Description of the Related Art
Crystal growth apparatuses or furnaces, such as directional solidification systems (DSS) and heat exchanger method (HEM) furnaces, involve the melting and controlled resolidification of a feedstock material, such as alumina or silicon, in a crucible to produce an ingot. Production of a solidified ingot from molten feedstock occurs in several identifiable steps over many hours. For example, to produce an ingot, such as a sapphire ingot, by the HEM method, solid feedstock, such as alumina, is provided in a crucible containing a monocrystalline seed (which comprises the same material as the feedstock but with a single crystal orientation throughout) placed into the hot zone of a solidification furnace. A heat exchanger, such as a helium-cooled heat exchanger, is positioned in thermal communication with the crucible bottom and with the monocrystalline seed. The feedstock is then heated to form a liquid feedstock melt, without substantially melting the monocrystalline seed, and heat is then removed from the melted feedstock by applying a temperature gradient in the hot zone in order to directionally solidify the melt from the unmelted seed. By controlling how the melt solidifies, a crystalline material having a crystal orientation corresponding to that of the monocrystalline seed, and having greater purity than the starting feedstock material, can be achieved.
However, during the melt phase, in some circumstances the seed melts beyond a predetermined threshold, causing an undesired or unexpected crystallographic structure. Therefore, to improve the error rate of such unexpected crystallographic structure, a melt back of the seed is usually measured. This measurement is often referred to as a seed melt back. These measurements are generally performed manually by illuminating the boule, such as from the side, top, or bottom, and using tools such as vernier calipers to estimate the melt back of the seed. However, manual measurements lead to human error and are typically very inefficient in crystalline material production.